Apparatus and method to compensate bearing runout in an articulated arm coordinate measurement machine

ABSTRACT

A method and apparatus for correcting errors in a bearing cartridge used in a portable articulated arm coordinate measurement machine (AACMM) is provided. The method includes providing a cartridge having a first bearing and a second bearing arranged in a fixed relationship to define an axis, the cartridge further including an angle measurement device configured to measure a rotation of a portion of the cartridge about the axis. A plurality of angles is measured with the angle measurement device. A first plurality of displacements is determined at a first position along the axis, each of the first plurality of displacements being associated with one of the plurality of angles. Compensation values are determined based at least in part on the plurality of angles and the first plurality of displacements.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.14/823,261 filed on Aug. 11, 2015. U.S. application Ser. No. 14/823,261is a continuation-in-part of U.S. application Ser. No. 14/610,052 filedon Jan. 30, 2015, which was nonprovisional application of U.S.Provisional Application Ser. No. 61/936,416 filed on Feb. 6, 2014. Thepresent application is further a continuation-in-part of U.S.application Ser. No. 14/726,873 filed on Jun. 1, 2015, which is acontinuation-in-part of U.S. application Ser. No. 13/888,442 filed onMay 7, 2013, now U.S. Pat. No. 9,075,025. U.S. Pat. No. 9,075,025 is anonprovisional application of U.S. Provisional Application Ser. No.61/647,697 filed on May 16, 2012. The present application is also acontinuation-in-part of U.S. application Ser. No. 14/729,151 filed onJun. 3, 2015, which is a nonprovisional application of U.S. ApplicationSer. No. 62/011,288 filed on Jun. 12, 2014. The contents of all of theabove are incorporated by reference herein in their entirety.

BACKGROUND

The present disclosure relates to a coordinate measuring machine, andmore particularly to an apparatus and method of determining the bearingrunout in cartridges of portable articulated arm coordinate measurementmachines.

Portable articulated arm coordinate measuring machines (AACMMs) havefound widespread use in the manufacturing or production of parts wherethere is a need to rapidly and accurately verify the dimensions of thepart during various stages of the manufacturing or production (e.g.,machining) of the part. Portable AACMMs represent a vast improvementover known stationary or fixed, cost-intensive and relatively difficultto use measurement installations, particularly in the amount of time ittakes to perform dimensional measurements of relatively complex parts.Typically, a user of a portable AACMM simply guides a probe along thesurface of the part or object to be measured. The measurement data arethen recorded and provided to the user. In some cases, the data areprovided to the user in visual form, for example, three-dimensional(3-D) form on a computer screen. In other cases, the data are providedto the user in numeric form, for example when measuring the diameter ofa hole, the text “Diameter=1.0034” is displayed on a computer screen.

An example of a prior art portable articulated arm CMM is disclosed incommonly assigned U.S. Pat. No. 5,402,582 ('582), which is incorporatedherein by reference in its entirety. The '582 patent discloses a 3-Dmeasuring system comprised of a manually-operated articulated arm CMMhaving a support base on one end and a measurement probe at the otherend. Commonly assigned U.S. Pat. No. 5,611,147 ('147), which isincorporated herein by reference in its entirety, discloses a similararticulated arm CMM. In the '147 patent, the articulated arm CMMincludes a number of features including an additional rotational axis atthe probe end, thereby providing for an arm with either a two-two-two ora two-two-three axis configuration (the latter case being a seven axisarm).

Relative rotational movement between the arm segments of the articulatedarm CMM typically involves cartridges having a pair of bearings and anangular encoder. Current calibration methods account for mechanicalerrors such as non-squareness and axis offset. However other errors,such as bearing runout, may cause deviations in the measurementsperformed by the articulated arm CMM.

Accordingly, while existing methods of manufacturing articulated armCMM's are suitable for their intended purposes the need for improvementremains, particularly in providing a method and apparatus for measuringbearing runout so compensation parameters may be determined.

BRIEF DESCRIPTION

In accordance with an embodiment, a method of correcting errors in abearing cartridge used in a portable articulated arm coordinatemeasurement machine (AACMM) is provided. The method comprising providingthe cartridge having a first bearing and a second bearing arranged in afixed relationship to define an axis, the cartridge further including anangle measurement device configured to measure an angle of rotation of aportion of the cartridge about the axis, the angle measurement devicebeing further configured to transmit an angle measurement signal inresponse to the rotation of the portion of the cartridge about the axis;rotating the portion of the cartridge about the axis by a plurality ofturns in a forward direction and in a reverse direction, each turn being360 degrees; measuring for each of the plurality of turns a plurality offirst rotational values, each of the plurality of first rotationalvalues comprising a first number of turns of the plurality of turns thatthe portion of the cartridge is rotated in the forward direction minus asecond number of turns of the plurality of turns the portion of thecartridge is rotated in the reverse direction; measuring over theplurality of turns a plurality of angles with the angle measurementdevice; determining a first plurality of displacements at a firstposition along the axis, each of the first plurality of displacementsbeing associated with one of the plurality of angles and one of theplurality of first rotational values; determining compensation valuesbased at least in part on the measured plurality of angles, theplurality of first rotational values, and the determined first pluralityof displacements; storing the compensation values in a memory; providingthe AACMM with the cartridge installed between two arm segments and arotational counter configured to measure a second rotational value ofthe installed cartridge, the second rotational value being a thirdnumber of turns the portion of the cartridge is rotated in the forwarddirection minus a fourth number of turns the portion of the cartridge isrotated in the reverse direction, wherein the rotational counter isfurther configured to measure the second rotational value when the AACMMis in a powered off-state and in a powered on-state; measuring athree-dimensional coordinate of an object with the AACMM based at leastin part on the angular measurement signal, the stored compensationvalues, and second rotational value.

These and other advantages and features will become more apparent fromthe following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWING

The subject matter, which is regarded as the invention, is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIGS. 1A and 1B are perspective views of a portable articulated armcoordinate measuring machine (AACMM) having embodiments of variousaspects of the present invention therewithin;

FIG. 2, including FIGS. 2A-2E taken together, is a block diagram ofelectronics utilized as part of the AACMM of FIG. 1 in accordance withan embodiment;

FIG. 3, including FIGS. 3A, 3B, 3C, 3D and 3E taken together, is a blockdiagram describing detailed features of the electronic data processingsystem of FIG. 2 in accordance with an embodiment;

FIG. 4 is a perspective view of the AACMM of FIG. 1;

FIG. 5 is a perspective view of the probe end of the AACMM of FIG. 1with a handle accessory being coupled thereto;

FIG. 6 is a partial exploded view illustrating a pair of encoder/bearingcartridges being assembled between two dual socket joints in accordancewith an embodiment;

FIG. 7 is an exploded perspective view illustrating of theencoder/bearing cartridge of FIG. 6;

FIG. 8 is a side sectional view of the cartridge of FIG. 6;

FIGS. 9A and 9B are perspective views of a prior art apparatus thatmeasures bearing errors;

FIG. 10A-C are plots of data obtained from a measurement of bearingerrors in a lathe spindle;

FIG. 11 shows four consecutive rotations of a spindle that contains twobearings;

FIG. 12 is perspective view, partially in section, of an encoder/bearingcartridge and a bearing runout measurement apparatus according to anembodiment of the invention;

FIG. 13 is a perspective view of an exemplary drive mechanism for usewith the bearing runout measurement apparatus of FIG. 10;

FIG. 14 is a top view of the drive mechanism of FIG. 13;

FIG. 15 is a perspective view of another embodiment of a drive mechanismfor use with the bearing runout measurement apparatus of FIG. 10;

FIG. 16 is a top view of the drive mechanism of FIG. 15;

FIG. 17 is perspective view of an encoder/bearing cartridge and abearing runout measurement apparatus according to another embodiment ofthe invention; and

FIG. 18A and FIG. 18B are schematic illustrations of the operation ofthe apparatus of FIG. 17.

The detailed description explains embodiments of the invention, togetherwith advantages and features, by way of example with reference to thedrawings.

DETAILED DESCRIPTION

An embodiment of the present invention provides an enhanced AACMM thatcompensates for bearing cartridge errors, such as bearing runout andtilt/wobble. The compensation values measured for the bearing cartridgesprovides advantages in enhancing accuracy of the coordinate pointsmeasured by the AACMM. Embodiments of the invention provide advantagesin a compensation system and method for measuring, recording and storingcompensation values for each bearing cartridge to accommodatesynchronous and asynchronous errors, such as bearing runout ortilt/wobble.

FIGS. 1A and 1B illustrate, in perspective, an AACMM 100 according tovarious embodiments of the present invention, an articulated arm beingone type of coordinate measuring machine. As shown in FIGS. 1A and 1B,the exemplary AACMM 100 may comprise a six or seven axis articulatedmeasurement device having a probe end 401 that includes a measurementprobe housing 102 coupled to an arm portion 104 of the AACMM 100 at oneend. The arm portion 104 comprises a first arm segment 106 coupled to asecond arm segment 108 by a rotational connection having a firstgrouping of bearing cartridges 110 (e.g., two bearing cartridges). Asecond grouping of bearing cartridges 112 (e.g., two bearing cartridges)couples the second arm segment 108 to the measurement probe housing 102.A third grouping of bearing cartridges 114 (e.g., three bearingcartridges) couples the first arm segment 106 to a base 116 located atthe other end of the arm portion 104 of the AACMM 100. Each grouping ofbearing cartridges 110, 112, 114 provides for multiple axes ofarticulated movement. Also, the probe end 401 may include a measurementprobe housing 102 that comprises the shaft of the seventh axis portionof the AACMM 100 (e.g., a cartridge containing an encoder system thatdetermines movement of the measurement device, for example a contactprobe 118, in the seventh axis of the AACMM 100). In this embodiment,the probe end 401 may rotate about an axis extending through the centerof measurement probe housing 102. In use of the AACMM 100, the base 116is typically affixed to a work surface.

Each bearing cartridge within each bearing cartridge grouping 110, 112,114 typically contains an encoder system (e.g., an optical angularencoder system). The encoder system (i.e., transducer) provides anindication of the position of the respective arm segments 106, 108 andcorresponding bearing cartridge groupings 110, 112, 114 that alltogether provide an indication of the position of the probe 118 withrespect to the base 116 (and, thus, the position of the object beingmeasured by the AACMM 100 in a certain frame of reference—for example alocal or global frame of reference). The arm segments 106, 108 may bemade from a suitably rigid material such as but not limited to a carboncomposite material for example. A portable AACMM 100 with six or sevenaxes of articulated movement (i.e., degrees of freedom) providesadvantages in allowing the operator to position the probe 118 in adesired location within a 360° area about the base 116 while providingan arm portion 104 that may be easily handled by the operator. However,it should be appreciated that the illustration of an arm portion 104having two arm segments 106, 108 is for exemplary purposes, and theclaimed invention should not be so limited. An AACMM 100 may have anynumber of arm segments coupled together by bearing cartridges (and,thus, more or less than six or seven axes of articulated movement ordegrees of freedom).

As will be discussed in more detail below, each of the groupings ofbearing cartridges 110, 112, 114 may include one or more slip rings221A-221D. The slip ring 221A-221D allows for the transfer of electricalalong the length of the arm portion 104 while still allowing each of thegroupings of bearing cartridges 110, 112, 114 to rotate substantiallyunencumbered.

The probe 118 is detachably mounted to the measurement probe housing102, which is connected to bearing cartridge grouping 112. A handleaccessory 126 is removable with respect to the measurement probe housing102 by way of, for example, a quick-connect interface. In the exemplaryembodiment, the quick-connect interface may include both mechanicalfastening members that secure the accessory to the housing 102 andelectrical connections that allow the user to control the probe 118through the accessory (e.g. actuation buttons) and also provide for highspeed data communication between the accessory and the base 116. Thehandle 126 may be replaced with another device (e.g., a laser lineprobe, a bar code reader), thereby providing advantages in allowing theoperator to use different measurement devices with the same AACMM 100.In exemplary embodiments, the probe housing 102 houses a removable probe118, which is a contacting measurement device and may have differenttips 118 that physically contact the object to be measured, including,but not limited to: ball, touch-sensitive, curved and extension typeprobes. In other embodiments, the measurement is performed, for example,by a non-contacting device such as a laser line probe (LLP). In anembodiment, the handle 126 is replaced with the LLP using thequick-connect interface. Other types of accessory devices may replacethe removable handle 126 to provide additional functionality. Examplesof such accessory devices include, but are not limited to, one or moreillumination lights, a temperature sensor, a thermal scanner, a bar codescanner, a projector, a paint sprayer, a camera, a video camera, anaudio recording system or the like, for example.

As shown in FIGS. 1A and 1B, the AACMM 100 includes the removable handle126 that provides advantages in allowing accessories or functionality tobe changed without removing the measurement probe housing 102 from thebearing cartridge grouping 112. As discussed in more detail below withrespect to FIG. 2, the removable handle 126 may also include one or moreelectrical connectors that allow electrical power and data to beexchanged with the handle 126 and the corresponding electronics locatedin the probe end 401 and the base 116.

In various embodiments, and as will be discussed in more detail below,each rotational connection includes a grouping of bearing cartridges110, 112, 114 that allow the arm portion 104 of the AACMM 100 to moveabout multiple axes of rotation. As mentioned, each bearing cartridgegrouping 110, 112, 114 includes corresponding encoder systems, such asoptical angular encoders for example, that are each arranged coaxiallywith the corresponding axis of rotation of, e.g., the arm segments 106,108. The optical encoder system detects rotational (swivel) ortransverse (hinge) movement of, e.g., each one of the arm segments 106,108 about the corresponding axis and transmits a signal to an electronicdata processing system within the AACMM 100 as described in more detailherein below. Each individual raw encoder count is sent separately tothe electronic data processing system as a signal where it is furtherprocessed into measurement data. No position calculator separate fromthe AACMM 100 itself (e.g., a serial box) is required, as disclosed incommonly assigned U.S. Pat. No. 5,402,582 ('582).

The base 116 may include an attachment device or mounting device 120.The mounting device 120 allows the AACMM 100 to be removably mounted toa desired location, such as an inspection table, a machining center, awall or the floor for example. In one embodiment, the base 116 includesa handle portion 122 that provides a convenient location for theoperator to hold the base 116 as the AACMM 100 is being moved. In oneembodiment, the base 116 further includes a movable cover portion 124that folds down to reveal a user interface, such as a display screen.

In accordance with an embodiment, the base 116 of the portable AACMM 100contains or houses an electronic data processing system that includestwo primary components: a base processing system that processes the datafrom the various encoder systems within the AACMM 100 as well as datarepresenting other arm parameters to support three-dimensional (3-D)positional calculations; and a user interface processing system thatincludes an on-board operating system, a touch screen display, andresident application software that allows for relatively completemetrology functions to be implemented within the AACMM 100 without theneed for connection to an external computer.

The electronic data processing system in the base 116 may communicatewith the encoder systems, sensors, and other peripheral hardware locatedaway from the base 116 (e.g., a LLP that can be mounted to or within theremovable handle 126 on the AACMM 100). The electronics that supportthese peripheral hardware devices or features may be located in each ofthe bearing cartridge groupings 110, 112, 114 located within theportable AACMM 100.

FIG. 2 is a block diagram of electronics utilized in an AACMM 100 inaccordance with an embodiment. The embodiment shown in FIG. 2A includesan electronic data processing system 210 including a base processorboard 204 for implementing the base processing system, a user interfaceboard 202, a base power board 206 for providing power, a Bluetoothmodule 232, and a base tilt board 208. The user interface board 202includes a computer processor for executing application software toperform user interface, display, and other functions described herein.

As shown in FIG. 2A-2D, the electronic data processing system 210 is incommunication with the aforementioned plurality of encoder systems viaone or more electrical buses 218A, 218B, 218C, 218D. It should beappreciated that the data processing system 210 may include additionalcomponents, such as connector 211, for example, that are configured toadapt the incoming and outgoing signals to an electrical bus 218A-218D.For the clarity purposes, not all of these components are shown in FIG.2. In the embodiment depicted in FIG. 2, each encoder system generatesencoder data and includes: an encoder bus interface 214, an encoderdigital signal processor (DSP) 216, an encoder read head interface 234,and a temperature sensor 212. Other devices, such as strain sensors, maybe attached to the electrical bus 218.

Also shown in FIG. 2E are probe end electronics 230 that are incommunication with the electrical bus 218E. The probe end electronics230 include a probe end DSP 228, a temperature sensor 212, a handle/LLPelectrical bus 240 and a bus 241 that connects with the handle 126 orthe LLP 242 via the quick-connect interface in an embodiment, and aprobe interface 226. The bus 241 may be an electrical bus, an opticalbus, or a bus that includes both optical and electrical signals. Thequick-connect interface allows access by the handle 126 to theelectrical bus 240 and bus 241 for the LLP and other accessories. Theelectrical bus may contain data lines, control lines, and power lines.The optical bus may contain data lines and control lines. In anembodiment, the probe end electronics 230 are located in the measurementprobe housing 102 on the AACMM 100. In an embodiment, the handle 126 maybe removed from the quick-connect interface and measurement may beperformed by the laser line probe (LLP) 242 communicating with the probeend electronics 230 of the AACMM 100 via the handle/LLP electrical bus240. It should be appreciated that while the electrical bus 218 isdiscussed as an individual component, the bus 218 may be formed from aplurality of individual bus segments (e.g. bus 218A-218E) that areserially connected to transfer signals within the AACMM 100. As isdiscussed in more detail herein, each segment may be separated by arotary cartridge (FIGS. 6-8) having an electrical slip ring 221A-221D.

In an embodiment, the electronic data processing system 210 is locatedin the base 116 of the AACMM 100, the probe end electronics 230 arelocated in the measurement probe housing 102 of the AACMM 100, and theencoder systems are located in the bearing cartridge groupings 110, 112,114. The probe interface 226 may connect with the probe end DSP 228 byany suitable communications protocol, including commercially-availableproducts from Maxim Integrated Products, Inc. that embody the 1-Wire®communications protocol 236.

FIGS. 3A-3C are block diagrams describing detailed features of theelectronic data processing system 210 (FIG. 2A) of the AACMM 100 inaccordance with an embodiment. In an embodiment, the electronic dataprocessing system 210 is located in the base 116 of the AACMM 100 andincludes the base processor board 204, the user interface board 202, abase power board 206, a Bluetooth module 232, and a base tilt module208.

In an embodiment shown in FIGS. 3A-3C, the base processor board 204includes the various functional blocks illustrated therein. For example,a base processor function 302 is utilized to support the collection ofmeasurement data from the AACMM 100 and receives raw arm data (e.g.,encoder system data) via the electrical bus 218 and a bus control modulefunction 308. The memory function 304 stores programs and static AACMMconfiguration data. The base processor board 204 also includes anexternal hardware option port function 310 for communicating with anyexternal hardware devices or accessories such as but not limited to agraphical monitor or television via HDMI port 311, an audio device viaport 313, a USB 3.0 port 315 and a flash memory (SD) card via port 317for example. A real time clock (RTC) and log 306, a battery packinterface (IF) 316, and a diagnostic port 318 are also included in thefunctionality in an embodiment of the base processor board 204 depictedin FIG. 3.

The base processor board 204 also manages all the wired and wirelessdata communication with external (host computer) and internal (displayprocessor 328) devices. The base processor board 204 has the capabilityof communicating with an Ethernet network via a gigabit Ethernetfunction 320 (e.g., using a clock synchronization standard such asInstitute of Electrical and Electronics Engineers (IEEE) 1588), with awireless local area network (WLAN) via a LAN function 322, and withBluetooth module 232 via a parallel to serial communications (PSC)function 314. The base processor board 204 also includes a connection toa universal serial bus (USB 3.0) device 312.

The base processor board 204 transmits and collects raw measurement data(e.g., encoder system counts, temperature readings) for processing intomeasurement data without the need for any preprocessing, such as in theserial box disclosed in the aforementioned '582 patent. The baseprocessor 204 sends the processed data to the display processor 328 onthe user interface board 202 via an RS485 interface (IF) 326. In anembodiment, the base processor 204 also sends the raw measurement datato an external computer.

Turning now to the user interface board 202 in FIG. 3, the angle andpositional data received by the base processor is utilized byapplications executing on the display processor 328 to provide anautonomous metrology system within the AACMM 100. Applications may beexecuted on the display processor 328 to support functions such as, butnot limited to: measurement of features, guidance and training graphics,remote diagnostics, temperature corrections, control of variousoperational features, connection to various networks, and display ofmeasured objects. Along with the display processor 328 and a liquidcrystal display (LCD) 338 (e.g., a touch screen LCD) user interface, theuser interface board 202 includes several interface options including amemory 332, a USB Host interface 334, a diagnostic port 336, a cameraport 340, an audio/video interface 342, a dial-up/cell modem 344 and aglobal positioning system (GPS) port 346.

The electronic data processing system 210 shown in FIG. 3 also includesa base power board 206 with an environmental recorder 362 for recordingenvironmental data. The base power board 206 also provides power to theelectronic data processing system 210 using an AC/DC converter 358 and abattery charger control 360. The base power board 206 communicates withthe base processor board 204 using inter-integrated circuit (I2C) serialsingle ended bus 354 as well as via a DMA serial peripheral interface(DSPI) 356. The base power board 206 is connected to a tilt sensor andradio frequency identification (RFID) module 208 via an input/output(I/O) expansion function 364 implemented in the base power board 206.

Though shown as separate components, in other embodiments all or asubset of the components may be physically located in differentlocations and/or functions combined in different manners than that shownin FIG. 3. For example, in one embodiment, the base processor board 204and the user interface board 202 are combined into one physical board.

Referring now to FIGS. 1 and 4, an embodiment is shown of the AACMM 100having an integrated display. The AACMM 100 includes a base 116 thatincludes the electronic data processing system 210 that is arranged tocommunicate with the electrical busses 218. Data carried by electricalbus 218 may come from encoders associated with the bearing cartridgegroups 110, 112, 114 or from arm accessories. The base 116 includes ahousing 399 with the mounting device 120 on one end and the bearingcartridge grouping 114 and arm portion 104 on an opposite end. On oneside, the housing 399 includes a recess 403. The recess is defined by aninterior wall 405, a first side wall 407, a second side wall 409 and anend wall 411. The side walls 407, 409 are arranged on an angle relativeto the mounting plane of the AACMM 100 such that the recess 403 tapersfrom the end adjacent the mounting device 120 to the end adjacent thearm portion 104. Adjacent the end wall 411, the housing 399 includes ahandle portion 122 that is sized to facilitate the carrying of the AACMM100 by the operator.

In one embodiment, the recess 403 includes an opening sized to receive abattery 414. The battery 414 is removably disposed in the housing 399and is secured by a latch 415 that is movably disposed in wall 405. Thelatch 415 may include a tab portion that engages a surface of thebattery 414 and prevents inadvertent removal. The battery 414 may becoupled to a battery pack interface and provide electrical power to theAACMM 100 when the AACMM 100 is not connected to an external powersource (e.g. a wall outlet). In the exemplary embodiment, the battery414 includes circuitry that communicates with the electronic dataprocessing system 210 and transmits signals that may include, but arenot limited to: battery charge level; battery type; model number;manufacturer; characteristics; discharge rate; predicted remainingcapacity; temperature; voltage; and an almost-discharged alarm so thatthe AACMM can shut down in a controlled manner.

Also disposed on wall 405 may be one or more external ports that arecoupled to electronic data processing system 210, such as flash memorycard port 317, USB 3.0 port 315, HDMI port 311 and audio port 313 forexample. The external ports are arranged to be accessible to the userwhen the movable cover portion 124 is moved from a closed position (FIG.1A) to an open position (FIG. 4).

The movable cover portion 124 includes a housing member 423 that ismounted to hinges that couple the movable cover portion 124 to the endwall 411. In the exemplary embodiment, when in the open position, themovable cover portion 124 is arranged at an obtuse angle relative to theinterior wall 404. It should be appreciated that the movable coverportion 124 is continuously rotatable and that the open position may beany position at which the operator can access and utilize the displayscreen.

The movable cover portion 124 further includes a face member 424disposed on one side and coupled to the housing member 423. The facemember 424 includes an opening 425 sized to allow the viewing of adisplay 428. The housing member 423 and face member 424 are generallythin wall structures, formed from an injection molded plastic materialfor example, that define a hollow interior portion. In one embodiment,the housing member 423 or face member 424 may be formed from othermaterials, including but not limited to steel or aluminum sheet metalfor example.

Arranged within the movable cover portion 124 is a display 428 having adisplay 428. The display 428 is mounted to the face member 424. Thedisplay 428 provides a user interface that allows the operator tointeract and operate the AACMM 100 without utilizing or connecting anexternal host computer. The display 428 may include a touch sensitivescreen having elements for detecting the touch that include, but are notlimited to: resistive elements; surface acoustic wave elements;capacitive elements; surface capacitance elements; projected capacitanceelements; infrared photodetector elements; strain gauge elements;optical imaging elements; dispersive signal elements; or acoustic pulserecognition elements. The display 428 is arranged in bidirectionalcommunication with the user interface board 202 and the base processorboard 204 such that actuation of the display 428 by the operator mayresult in one or more signals being transmitted to or from the display428.

Referring now to FIG. 5, an exemplary embodiment of a probe end 401 isillustrated having a measurement probe housing 102 with a quick-connectmechanical and electrical interface that allows removable andinterchangeable device 400 to couple with AACMM 100. In the exemplaryembodiment, the device 400 includes an enclosure 402 that includes ahandle portion 404 that is sized and shaped to be held in an operator'shand, such as in a pistol grip for example. The enclosure 402 may be athin wall structure having a cavity that houses a controller (notshown). The controller may be a digital circuit, having a microprocessorfor example, or an analog circuit. In one embodiment, the controller isin asynchronous bidirectional communication with the electronic dataprocessing system 210 (FIGS. 2 and 3). The communication connectionbetween the controller and the electronic data processing system 210 maybe a wireless, a wired (e.g. via bus 218) or an optical connection. Thecommunication connection may also include a direct or indirect wirelessconnection (e.g. Bluetooth or IEEE 802.11) or a combination of wired,optical and wireless connections. In the exemplary embodiment, theenclosure 402 is formed in two halves, such as from an injection moldedplastic material for example. The halves may be secured together byfasteners, such as screws for example. In other embodiments, theenclosure halves may be secured together by adhesives or ultrasonicwelding for example.

The handle portion 404 also includes buttons or actuators 416 that maybe manually activated by the operator. The actuators 416 may be coupledto the controller that transmits a signal to a controller within theprobe housing. In the exemplary embodiments, the actuators 416 performthe functions of actuators 422 located on the probe housing 102 oppositethe device 400. It should be appreciated that the device 400 may haveadditional switches, buttons or other actuators that may also be used tocontrol the device 400, the AACMM 100 or vice versa. Also, the device400 may include indicators, such as light emitting diodes (LEDs), soundgenerators, meters, displays or gauges for example. In one embodiment,the device 400 may include a digital voice recorder that allows forsynchronization of verbal comments with a measured point. In yet anotherembodiment, the device 400 includes a microphone that allows theoperator to transmit voice activated commands to the electronic dataprocessing system 210.

In one embodiment, the handle portion 404 may be configured to be usedwith either operator hand or for a particular hand (e.g. left handed orright handed). The handle portion 404 may also be configured tofacilitate operators with disabilities (e.g. operators with missingfinders or operators with prosthetic arms). Further, the handle portion404 may be removed and the probe housing 102 used by itself whenclearance space is limited. As discussed above, the probe end 401 mayalso comprise the shaft of the seventh axis of AACMM 100. In thisembodiment the device 400 may be arranged to rotate about the AACMMseventh axis.

In one embodiment, the probe end 401 includes a mechanical andelectrical interface that cooperates with a second connector on theprobe housing 102. The connectors may include electrical and mechanicalfeatures that allow for coupling of the device 400 to the probe housing102, such as that described in commonly owned U.S. Pat. No. 8,533,967entitled “Coordinate Measurement Machines with Removable Accessories,”which is incorporated herein by reference in its entirety. Thiselectrical and mechanical interface provides for a relatively quick andsecure electronic connection between the device 400 and the probehousing 102 without the need to align connector pins, and without theneed for separate cables or connectors.

The probe housing 102 includes a collar 438 arranged co-axially on oneend. The collar 438 includes a threaded portion that is movable betweena first position and a second position. By rotating the collar 438, thecollar 438 may be used to secure or remove the device 400 without theneed for external tools. In one embodiment, rotation of the collar 438moves the collar 438 along a relatively coarse, square-threaded cylinder(not shown). The use of such relatively large size, square-thread andcontoured surfaces allows for significant clamping force with minimalrotational torque. The coarse pitch of the threads further allows thecollar 438 to be tightened or loosened with minimal rotation.

The coupling of the probe end 401 to the end of the arm portion 104creates a communication connection between the bus 218 and thetransceiver 421. This coupling further creates a communicationconnection between the contact probe 118 and the electronic dataprocessing system 210. In this manner, signals may be transmitted andreceived over bus 218. It should be appreciated that it is desirable forthe segments 106, 108 of the arm portion 104 and the probe end 401 to berotatable on several axis of rotation to allow the probe end 401 to bepositioned to make a desired measurement without inhibiting the user. Asa result, one or more electrical connections are made at each of thebearing cartridge groupings 110, 112, 114 for each rotational joint.These connections allow the arm portion 104 to be moved and rotatedwithout interference from electrical conductors or optical conductors.

Referring now to FIGS. 6-8, an exemplary embodiment is shown of an armrotational connection using groupings of bearing cartridges, such asbearing cartridge grouping 110 for example, that include a slip ringassembly that allows for rotation of the arm segments. As discussedabove, each of the rotational connections of the articulated armutilizes a modular bearing/encoder cartridge such as cartridge 500 orcartridge 502 for example. These cartridges 500, 502 are mounted in theopenings of dual socket joints 504, 506. Each socket joint 504, 506includes a first cylindrical extension 508 having a first recess orsocket 510 and a second cylindrical extension 512 having a second recessor socket 514. Generally sockets 510, 514 are positioned 90° to oneanother although other relative angular configurations may be employed.Cartridge 502 is positioned in each socket 516 of dual socket joints504, 506 to define a hinge joint, while cartridge 500 is positioned insocket 510 of joint 506 to define a longitudinal swivel joint. Modularbearing/encoder cartridges 500, 502 provide advantages in permittingseparate manufacturer of a pre-stressed or preloaded dual bearingcartridge on which is mounted the modular encoder components. Thisbearing encoder cartridge can then be fixedly attached to the externalskeletal components, such as dual socket joints 504, 506 for example, ofthe articulated arm portion 104. The use of such cartridges isadvantageous in permitting high-quality, high-speed production of thesesophisticated subcomponents of articulated arm portion 104.

In some embodiments, there may be as many as four or more differentcartridge types, for example two “long” axial cartridges that allow forswivel rotation, and two “short” cartridges that provide a hinge joint.Each cartridge includes a pre-loaded bearing arrangement and atransducer which may comprise a digital encoder. While the length of thecartridge may change, for exemplary purposes, we will describe all typesof cartridges with respect to cartridge 500.

The cartridge 500 includes a pair of bearings 600, 602 separated by aninner sleeve 604 and an outer sleeve 606. It is desirable that thebearings 604, 606 be pre-loaded. In one embodiment, the preload isprovided by the sleeves 604, 606 being different lengths (inner sleeve604 is shorter than the outer sleeve 606 by approximately 0.0005 inches)so that upon tightening, a preselected preload is generated on bearings600, 602. Bearings 600, 602 may be sealed using seals 608 and rotatablymounted on shaft 610. At its upper surface, shaft 610 terminates at ashaft upper housing 612. An annulus 614 (FIG. 8) is defined betweenshaft 610 and shaft upper housing 612. This entire assembly 600, 602,608, 610 is positioned within outer cartridge housing 616 with the shaft610 and its bearing assembly 600, 602 being securely attached to housing616 using a combination of an inner nut 618 and an outer nut 621. In oneembodiment, upon assembly the upper portion 622 of housing 616 may bereceived within annulus 614. It should be appreciated that theaforementioned preload is provided to bearings 600, 602 upon thetightening of the inner and outer nuts 618, 621 which providecompression forces to the bearings and, due to the difference in lengthbetween the sleeves 604, 606, the desired preload will be applied.

In the exemplary embodiment, bearings 600, 602 are duplex ball bearings.In order to obtain the desired level of pre-load, it is desired that theend surfaces of the bearings be parallel. This parallelism affects theevenness of the pre-loading about the circumference of the bearing.Uneven loading may give the bearing a rough, uneven running torque feeland may result in radial run out and reduced encoder performance. Radialrun out of the modularly mounted encoder disk may result in anundesirable fringe pattern shaft beneath the encoder head, which canresult in encoder angular measurement errors. As discussed in moredetail below, errors due to the radial run out may be measured andcompensated for once the cartridge 500 has been assembled.

The angular error of the cartridge 500 is directly related to theseparation of the bearings 600, 602. The angular error decreases as theseparation of the bearings 600, 602 increases. The sleeves 604, 606 maybe used to enhance the separation of the bearings 600, 602. In oneembodiment, the cartridge housing 616 and the sleeves 604, 606 are madefrom aluminum and are precision machined in length and parallelism. As aresult, changes in temperature should not result in differentialexpansion which could compromise pre-load. As previously mentioned, thepre-load is established by the difference in length between the sleeves604, 606. Once the nuts 618, 620 are fully tightened, this differentialin length will result in the desired bearing pre-load. The use of seals608 provide sealed bearings since contaminants would affect rotationalmovement and encoder accuracy, as well as joint feel.

It should be appreciated that while cartridge 500 is illustrated ashaving a pair of spaced bearings, the cartridge 500 may include a singlebearing or three or more bearings. Thus, each cartridge includes atleast one bearing.

In the exemplary embodiment, the cartridges may have unlimited rotation.In other embodiments, the cartridge may be limited to rotation over adefined angular range. For limited rotation, a groove may be formed on aflange 622 on the outer surface of housing 616, which provides acylindrical track to receive a shuttle 624. The shuttle 624 rides withinthe groove until it abuts a removable shuttle stop, such as a set screw(not shown), whereupon further rotation is precluded. The amount ofrotation can vary depending on what is desired. In one embodiment, theshuttle rotation is limited to less than 720 degrees.

In other embodiments, the cartridge may be configured for unlimitedrotation. In this latter case, a slip-ring assembly 627 is used. In oneembodiment, the shaft 610 has a hollow or axial opening 626therethrough, which has a larger diameter section 628 at one end. On oneend of the axial opening 628 is a slip ring assembly 627. The slip ringassembly 627 may consist of any commercially available slip ring, in oneembodiment, the slip ring assembly 627 comprises an H-series slip ringavailable from IDM Electronics Ltd. of Reading, Berkshire, UnitedKingdom. The slip ring assembly is non-structural with respect to thepreloaded bearing assembly. The slip ring assembly 630 provides nomechanical function but only provides electrical or signal transferfunctions. Axial opening 628 at an aperture 629 which communicates witha channel 631 sized and configured to receive wiring 634 from the slipring assembly 627. Such wiring is secured in place and protected by awire cover 630, which is snapped onto and is received into the channeland aperture.

As discussed herein, cartridge 500 includes both a preloaded bearingstructure and an optical encoder structure. In the exemplary embodiment,the optical encoder structure includes a read head 636 and a gratingdisk 638. In this embodiment, a pair of read heads 636 is positioned ona read head connector board 640. Connector board 640 is attached viafasteners 642 to a mounting plate 644. Grating disk 638 is attached tothe lower surface of shaft 610, such as with an adhesive for example,and is spaced apart from and in alignment with read heads 636. A wirefunnel 646 and sealing cap 632 provide the final outer covering to theend of housing 616. Wire funnel 646 captures and retains the wiring 634.It should be appreciated that the encoder head disk 638 will be retainedand rotate with shaft 610. It should be further appreciated that whilethe illustrated embodiment shows two read heads 636, more than two readheads or a single read head may alternatively be used. Still further, inother embodiments, the positions of the read head 636 and the gratingdisk 638 may be reversed whereby the read head 636 rotates with theshaft 610.

As discussed above, bearing run out errors may result in errors angularrotations of the cartridge 500 as determined by the encodermeasurements. It should be appreciated that it is desirable to reduce orsubstantially eliminate such errors to obtain higher levels of accuracyin the AACMM measurements. Referring now to FIGS. 9A and 9B, a prior artapparatus 700 is shown for measuring bearing runout error. The apparatus700 includes a rotating assembly 702 and a fixed assembly 705. Therotating assembly 702 includes a first shaft portion 704, a second shaftportion 706, a first sphere portion 708 and a second sphere portion 710.The first shaft portion 704 has a surface 712 that attaches to atransfer element (not shown), which is then attached to a rotatingstructure under test. In one embodiment, the spheres are lapped to aform error of 50 nanometers or less. The first sphere portion 708 has afirst equator 714 that is a great circle of the sphere and is alignedperpendicular to the first and second shaft portions 704, 706. Thesecond sphere portion 710 has a first equator 716 that is a great circleof the sphere and is also aligned perpendicular to the first and secondshaft portions 704, 706. The fixed assembly 705 includes a frame 718 anda plurality of capacitive sensors 720-728 that are rigidly affixed tothe frame 718. Electrical connections 730-738 connect to the sensors720-728 and transmit signals to an external electrical circuit orprocessing system (not shown). In an embodiment, capacitive sensors 720,722 are aligned perpendicular to the first sphere portion 708 at thefirst equator 714. The capacitive sensors 724, 726 are alignedperpendicular to the second sphere portion 710 at the first equator 716.The capacitive sensors 720, 722 are spaced apart from the first sphereportion 708 to reduce the risk of collision with the sensors 724, 726during operation. The capacitive sensor 708 is positioned 90 degreesfrom capacitive sensor 722. The fixed assembly 705 is attached to anon-rotating structure. In an embodiment, the frame 718 is attached tothe fixed structure that holds the housing (stator) of the cartridge(spindle).

In one embodiment, capacitive sensors 724, 726 are also spaced about 20micrometers from the second sphere portion 710. The capacitive sensor724 is positioned 90 degrees from the capacitive sensor 726. In theexemplary embodiment, the capacitive sensor 728 is aligned co-axiallywith the second sphere portion 710 and the second shaft portion 706. Inanother embodiment, the capacitive sensor 728 is not provided in theapparatus 700.

In another embodiment, the rotating portion 702 includes a singlecylindrical shaft without any sphere portions. Such a cylindrical shaftmay be machined, coated with a nickel, and ground to a form error(cylindricity) of 50 nm or less. The capacitive sensors may bepositioned directly within about 20 micrometers of the cylindrical shaftas in the earlier embodiment. In other embodiments, shapes besides thecylindrical shaft or cylindrical shaft with spherical portions are used.

In another embodiment, only two capacitive sensors, for example, thesensors 722, 726 or the sensors 720, 724, are used. Because each of thecapacitive sensors captures information for the full 360 degreerotation, in principle, two properly aligned capacitive sensors providecomplete runout information.

Referring now to FIG. 9B, the apparatus 700 is illustrated depicting theaxis of rotation z and an angle of rotation θ. The angle θ is taken withrespect to an axis x perpendicular to the z axis. The first sphereportion 708 has a first frame of reference 740 that includes an origin742, which is located at the center of the spherical surface of thefirst sphere portion 708. The first frame of reference 740 has an axisz1 aligned with the axis of the first and second shaft portions 704, 706and with the axis z. The axis x1 is aligned with the capacitive sensor720 and the axis y1 is aligned with capacitive sensor 722. The axes x1,y1, z1 are mutually perpendicular.

The second sphere portion 710 has a second frame of reference 744 thatincludes an origin 746 at the center of the spherical surface of thesecond sphere portion 710. The second frame of reference 744 has an axisz2 aligned with the axis of the first and second shaft portions 704, 706and with the axis z. The axis x2 is aligned with the capacitive sensor724 and the axis y2 is aligned with capacitive sensor 726. The axes x2,y2, z2 are mutually perpendicular. The capacitive sensor 728 is alignedwith the z axis near the bottom of the second sphere portion 710. Thedistance between the first origin 740 and the second origin 744 alongthe z axis is the distance L.

For the embodiment in which a single cylindrical shaft is used in placeof a cylindrical and sphere combination as in FIGS. 9A, 9B, the sameaxes x1, y1, z1 and x2, y2, z2 are still used, with the originscorresponding to 742, 746 along the axis of symmetry of the cylindricalshaft. The axes may also be for the case in which only two capacitivesensors, for example 722, 726 or 720, 724, are present.

For each angle θ, the apparatus 700 measures five displacementsassociated with each of the capacitive sensors 720-728. Thesedisplacements are Δx1, Δy1, Δx2, Δy2 and Δz, respectively. From thesedisplacements, tilt angles αx and αy resulting from the bearing errorsmay be obtained:αx=(Δx1−Δx2)/L,  (1)αx=(Δy1−Δy2)/L,  (2)

In the past, bearing calibration techniques have been used mostly formeasuring high speed spindles of precision machining tools, such asdiamond turning machines, lathes, milling machines, grinders and thelike. Usually bearing calculations are performed first to ensure that amachine tool meets specifications and second to find ways to changemachine tool design to improve tool performance. Since machine toolscannot be adjusted while machining operations are performed, it isusually not possible to correct the behavior of the machine tools whilemachining operations are underway.

For any 360 degree rotation of a quality bearing, it is usually the casethat bearing error repeats its motion almost exactly as a function ofthe rotation angle θ. In other words, if the bearing is moved back andforth over the same 360 degree window, the pattern of errors recorded bythe capacitive sensors is almost the same for any give angle θ. However,for the most part bearing errors do not repeat over different cycles of360 degrees. This behavior is explained in a tutorial on “PrecisionSpindle Metrology” presented by Eric R. Marsh at the annual meeting ofthe American Society for Precision Engineering accessed from theinternet site http://www.scribd.com/doc/132020851/Spindle-Tutorial on 2May 2013, the contents of which are incorporated by reference herein inits entirety.

FIG. 10A is a plot 750 of data 752 obtained from a measurement ofbearing errors in a lathe spindle. The plot shows data obtained from asingle capacitive sensor in an arrangement similar to that of FIGS. 9Aand 9B but with a single sphere rather than five spheres. The maximumvalues observed in the 32 turns of the shaft are seen to be to liegenerally within the range of +/−600 nm. An observation that can beimmediately made from the plot is that the measured values are differentfor each of the 32 turns of the shaft.

FIG. 10B is a plot 754 of data 756 for three cycles within the box 758in FIG. 10A. A sinusoidal curve 760 is fit to the data 756 and theaverage of the sinusoidal curve is extracted as line 762. The sinusoidalcurve results largely from the difficulty in perfectly centering thefirst sphere portion 708 and the second sphere portion 710 on the axisof rotation. Because it is generally not possible to perfectly centerthese spheres on the axis of rotation, the fundamental sinusoidalcomponent is removed during processing of collected data. FIG. 10C is aplot 764 of the bearing error 766, obtained by subtracting the values ofthe sinusoid 760 from the measured data 756. The subtracting thefundamental sinusoidal component from the collected data is performedonly on the capacitive sensors 720-726, which measure radial(side-to-side) displacements, and not on capacitive sensor 728, whichmeasures axial displacement. For axial displacement, the fundamentalsinusoidal variation is meaningful and is not subtracted from thecollected data.

In general, bearings do not return to their initial displacement after arotation of 360 degrees. This effect is illustrated in FIG. 11, whichshows four consecutive rotations of a spindle that contains twobearings. Turn one begins in the rightmost direction at 0 degrees withan error of between 0 and −1 micrometers. It rotates counterclockwiseand after 360 degrees has an error of between 0 and +1 micrometer. Theerror at an angle of zero degrees for the second turn is the same as theerror at 360 degrees for the first turn. By studying the four turns, itcan be seen that no two of the turns has the same errors. These resultsdispel an often held notion that bearing error patterns repeat every 720degrees.

Referring now to FIG. 12, an embodiment is illustrated of a cartridge500 coupled to the bearing measurement apparatus 700. The bearingmeasurement apparatus 700 is coupled to an electrical or processingcircuit 800. The first shaft portion 704 is configured to rigidly attachto the axial opening 626 (FIG. 8) of cartridge 500, allowing no relativemovement between 704 and 626. The arrow 802 indicates an attachmentlocation. A transfer element 806 (FIGS. 13-16) may be added to join thefirst shaft portion 704 to the shaft 610. As will be discussed in moredetail below, a drive system 804 is coupled to rotate the shaft 610. Thedrive system 804 is configured to balance the torque to the shaft 610and also isolate error motions in the drive mechanism from the shaft610. The housing 616 of cartridge 500 is configured to rigidly attach toframe portion 718 of measuring apparatus 700, to reduce or minimize therelative motion between housing 616 and frame portion 718.

Bearing errors are generally very repeatable over any 360 degreeinterval. However, there may be significant variations over different360 degree intervals. To substantially eliminate bearing errors, it ishelpful to limit the range of rotation of the shaft 610 to those angularregions for which bearing calibration data has been taken and to keeptrack of the rotation angle of the axles during operation. Further, inthe exemplary embodiment keeping track of current 360 degree rotationinterval is performed even when the angular encoder sensor power is off.In an embodiment, this is done by associating a non-volatile rotationmonitor to the shaft 610. In one embodiment, the rotation monitor is atachometer 801, such as a Hall Effect sensor for example. The tachometer801 is electrically coupled to the processing circuit 800 duringcalibration. The sensor includes a target 803, such as a magnet forexample, attached to the shaft 610, such as on the inside surface ofaxial opening 626. Each time the target 803 passes the sensor 801, thesensor 801 produces a signal that indicates the direction of movement.An rotational counter, such as an electrical or mechanical counter,keeps track of the number of revolutions. It should be appreciated thatmany different physical quantities may be measured by the sensors 801,such as but not limited to capacitance, inductance, magnetism and light.In one embodiment, if rotation is outside of the range which bearingcalibration data has been taken, the AACMM may provide a warning messageto the user. Electrical signals from the sensor 801 may be transmittedto a circuit board 805 for processing. The circuit board 810 may utilizepower from the base power board 206 during normal operation and includea battery to provide non-volatile operation of the rotation monitor whenpower from the base power board 206 is not available.

In another embodiment, the sensor may be a mechanical sensor thatresponds mechanically to rotation of the axle and keep track of thecurrent 360 degree range of the axle. The mechanical sensor may countthe number of axle rotations without requiring electrical power for itsoperation. For example, springs may be used to provide a measurableamount of tension correlated to the number of rotations of the shaft610. In one embodiment, stops are used to limit the range of rotation.

In an embodiment where a mechanical sensor is used, an operator may setthe mechanical counter to a starting position whenever power isreapplied. Thereafter, the angular encoders keep track of the rotationcount about their axles. Many types of mechanical counters may be used.In an embodiment, the mechanical counter is a Geneva drive, which is agear mechanism that translates a continuous rotation into anintermittent rotary motion.

Some angle measuring devices, such as angular encoders for example, areconfigured to measure between 0 and 360 degrees. To keep track of theoverall rotation angle, it is customary to speak of unwrapped angles.For example, an angle that drops between 0 degrees, say to −10 degreesfor example, has a wrapped angular reading of 350 degrees but anunwrapped value of −10 degrees. Similarly an angle that exceeds 360degrees by 10 degrees would have a wrapped angular reading of 10 degreesand an unwrapped value of 370 degrees.

The rotation monitor, such as sensor 801 for example, may be abidirectional counter, which means that it keeps track of the number offorward counts and reverse counts. An axle that completes five rotationsin the forward direction and two rotations in the reverse direction hasa rotational value of 5−2=3 rotations in the forward direction relativeto an origin or home position. The net rotational value may be combinedwith the angle between 0 and 360 degrees measured by an angularmeasuring device such as an angular encoder to obtain the unwrappedangle: unwrapped angle=wrapped angle+(net rotational value)(360), whereit is understood that the net rotational value may be positive ornegative.

In one embodiment, the during the calibration process described herein,the cartridge is rotated a sufficiently large number of rotations (e.ggreater than 10 to 100 revolutions) in both the forward and reversedirections from the origin or home position to allow the operator to usethe AACMM and maintain the cartridge within the predetermined number ofrotations of the home position during operation. In one embodiment, thenumber of rotations in the forward direction (e.g. +100 rotations) andreverse direction (e.g. −100 rotations) used in the calibration processis stored in memory on the AACMM. In this embodiment, an alarm may beemitted to alert the operator when the net rotational value isapproaching or exceeds the number of rotations used in the calibrationprocess in the forward direction (e.g. +100 rotations) or is approachingor less than the number of rotations in the reverse direction (e.g. −100revolutions). It should be appreciated that while embodiments hereindescribe the number of revolutions used in the calibration process inthe forward and reverse directions as being the same, this is forexemplary purposes and the claimed invention should not be so limited.In other embodiments, the calibration process may use a different numberof revolutions in the forward and reverse directions.

It should be appreciated that the rotation value and angle of thecartridge shaft is associated with the measured displacements measuredby the bearing measurement apparatus 700. As a result, a compensationvalue or a plurality of compensation values may be determined for agiven bearing cartridge for each angle of rotation and each rotationalvalue that is measured during the calibration process. Thesecompensation values from each of the bearing cartridges in the AACMM 100may then be stored in memory, such as memory function 304 or a flashmemory card for example. The stored compensation values may be utilizedby the electronic data processing system 210 to account for bearingrunout errors, the tilt/wobble errors and improve the accuracy of thethree-dimensional coordinate values measured by the AACMM 100.

During the calibration process, the shaft 610 is driven by the drivesystem 804 as shown in FIGS. 13-14 via transfer element 806. As theshaft 610 is rotated, the first shaft portion 704 of bearing measurementapparatus 700 is rotated. It should be appreciated that the transferelement 806 may be any suitable coupling mechanism or device thesecurely and rigidly couples the shaft 610 to the first shaft portion704. In the exemplary embodiment, the drive system 804 includes a firstdrive belt 808 that extends between a first pulley assembly 810 and asecond pulley assembly 812. The first drive belt engages the transferelement 806 and operates to rotate the transfer mechanism via thefrictional coupling between the first drive belt 808 and a surface (e.g.the outer surface) of the transfer element 806. In other embodiments,the first drive belt 808 may include teeth that engage teeth on thetransfer element 806 and the pulley assemblies 810, 812. A pair oftensioners 814, 816 are positioned intermediate the transfer element 806and the respective pulley assemblies 810, 812. The tensioners 814, 816are moved by actuators 818, 820 in response to a signal from forcesensors 822, 824. The force sensors monitor the tension on the firstdrive belt 8080 and provide a desired level of tension via a closed loopfeedback to the actuators 818, 820 to prevent or reduce the risk ofslippage between the first drive belt 808 and the transfer element 806.

Each of the pulley assemblies 810, 812 includes an input pulley 826 andan output pulley 828. Each output pulley 828 is coupled to the firstdrive belt 808 as described above. The input pulley 826 is coupled tothe output pulley 828 by a shaft 830 such that rotation of either pulleycauses a rotation of the other. A motor 832 is provided with a pulleymember 834. The pulley member 834 is coupled to the input pulley 826first pulley assembly 810 via a second drive belt 836. The pulley member834 is further coupled to the input pulley 826 of the second pulleyassembly 812 via a third drive belt 838. In this manner, the motor 832is operably coupled to rotate the pulley assemblies 810, 812 andtherefore the transfer element 806. In the exemplary embodiment, themotor 832 is coupled to the frame 718. As discussed above, the frame 718and the cartridge housing 616 are rigidly affixed to a fixture (notshown). It should be appreciated that in other embodiments, the motor832 may be mounted to the fixture (or another frame) separately from theframe 718. It should be appreciated that the use of the drive belts 808,836, 838 facilitates the isolation of the cartridge 500 from motionerrors in the motor 832 and also ensures that a balanced force isapplied to the shaft 610, preventing or reducing the risk of a bendingmoment from being applied to cartridge shaft 610.

Referring now to FIGS. 15-16, another embodiment of drive system 804 isshown. This embodiment is similar to that of FIGS. 13-14, with a motor832 rotating a pulley 834 to drive a pair of drive belts 836, 838 thatare coupled to pulley assemblies 810, 812. The rotation of the drivebelts 836, 838 in turn moves drive belt 808 that is coupled between thepulley assemblies 810, 812 and the transfer element 806. In thisembodiment, the tensioner pulleys 814, 816 are replaced with pairs ofidler pulleys 840, 842. To tension the drive belt 808, an actuator oradjustable spring 844 is coupled to move the second pulley assembly 812toward or away from the transfer element 806. It should be appreciatedthat movement of the second pulley assembly 812 will in turn reduce orincrease the amount of tension on the drive belt 808. A second actuatoror adjustable spring 846 moves the motor 832 and pulley 834 along anarcuate path represented by arrow 848 to maintain the desired tension onthe drive belts 836, 838. The arcuate path 848 has a center coaxial withpulley assembly 810. As discussed above, the drive belts 808, 836, 838cooperate to isolate the motion errors from the motor 832 and thearrangement of drive mechanism 804 also ensures a balanced force coupleis applied to the transfer element 806, preventing or reducing the riskof a bending moment from being applied to cartridge shaft 610.

Referring now to FIG. 17, another embodiment of a bearing measurementapparatus 850 is shown. In this embodiment, a reflective surface, suchas mirror 852, is mounted to the end of shaft 610. Arranged opposite themirror 852 is an autocollimator 854. An autocollimator is an opticalinstrument for non-contact measurement of angles. An autocollimatoroperates by projecting an image onto a target mirror, and measuring thedeflection of the returned image against a scale by means of anelectronic detector. As shown in FIG. 18A, if the cartridge 500 has noor very little “wobble”, the image 856 from the autocollimator 854 isreflected back along the same or substantially the same path 858. Wherethe shaft 610 has angular displacement as it is rotated, the image 856will reflect back along a path 860 as shown in FIG. 18B that is on anangle θ to the image 856. The autocollimator 854 transmits a signal tothe electrical or processing circuit 800. As discussed above, the sensor801 may transmit a signal via a circuit board 805 to the processingcircuit 800. From the angle θ and the known distance between the mirror852 and the autocollimator 854, the displacement (the tilt angle) of thecartridge shaft 610 with respect to the cartridge housing 616 may bedetermined. During operation, as the shaft 610 is rotated, theprocessing circuit combines the signals from the autocollimator 854, andoptical encoder to generate a map of displacement as a function ofrotation over each 360 degree cycle.

It should be appreciated that the calculated displacements from theautocollimator measurements may be associated with the rotational valuesand the angle of the cartridge shaft. Thus compensation values may bedetermined for any angle of each rotational value that is measuredduring the calibration process. These compensation values from each ofthe bearing cartridges in the AACMM 100 may then be utilized by theelectronic data processing system 210 to account for bearing runouterrors and improve the accuracy of the three-dimensional coordinatesmeasured by the AACMM 100.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims. Moreover, the useof the terms first, second, etc. do not denote any order or importance,but rather the terms first, second, etc. are used to distinguish oneelement from another. Furthermore, the use of the terms a, an, etc. donot denote a limitation of quantity, but rather denote the presence ofat least one of the referenced item.

What is claimed is:
 1. A method of correcting errors in a bearingcartridge used in a coordinate measurement device, the methodcomprising: providing the cartridge having a first bearing and a secondbearing arranged in a predetermined relationship to define an axis, thecartridge further including an angle measurement device that measures anangle of rotation of a portion of the cartridge about the axis andtransmits an angle measurement signal in response; rotating the portionof the cartridge about the axis by a plurality of turns in a forwarddirection and in a reverse direction, each turn being 360 degrees;measuring for each of the plurality of turns a plurality of firstrotational values, each of the plurality of first rotational valuescomprising a first number of turns of the plurality of turns that theportion of the cartridge is rotated in the forward direction minus asecond number of turns of the plurality of turns the portion of thecartridge is rotated in the reverse direction; measuring over theplurality of turns a plurality of angles with the angle measurementdevice; determining a first plurality of displacements at a firstposition along the axis, each of the first plurality of displacementsbeing associated with one of the plurality of angles and one of theplurality of first rotational values; determining compensation valuesbased at least in part on the measured plurality of angles, theplurality of first rotational values, and the determined first pluralityof displacements; and storing the compensation values in a memory of thecoordinate measurement device.
 2. The method of claim 1, furthercomprising: providing a rotational counter that measures a secondrotational value of the cartridge, the second rotational value being athird number of turns the portion of the cartridge is rotated in theforward direction minus a fourth number of turns the portion of thecartridge is rotated in the reverse direction; and measuring athree-dimensional coordinate of an object with the coordinatemeasurement device based at least in part on the angular measurementsignal, the stored compensation values, and the second rotational value.3. The method of claim 1, wherein the determining the first plurality ofdisplacements includes measuring the first plurality of displacementsalong a first line substantially perpendicular to the axis.
 4. Themethod of claim 3, further comprising measuring a second plurality ofdisplacements at a second position along the axis, each of the secondplurality of displacements being associated with one of the plurality ofangles, the second displacements being measured along a second linesubstantially perpendicular to the axis, wherein the first position andthe second position are separated by a first distance.
 5. The method ofclaim 4, wherein the step of determining compensation values includesdetermining the compensation values further based at least in part onthe second plurality of displacements and the first distance.
 6. Themethod of claim 5, further comprising: providing a test apparatusconfigured to be removably attached to the cartridge, the test apparatushaving a first sensor and a second sensor; and attaching the testapparatus to the cartridge so as to arrange the first sensor at thefirst position and the second sensor at the second position; wherein inthe step of measuring the plurality of first displacements, the firstdisplacements are measured with the first sensor, and in the step ofmeasuring the plurality of second displacements, the seconddisplacements are measured by the second sensor.
 7. The method of claim1, further comprising: providing a reflective surface attached to oneend of the cartridge and configured to rotate about the axis with theportion; providing an autocollimator; emitting a light beam from theautocollimator toward the reflective surface; receiving by theautocollimator a reflected beam, the reflected light beam being aportion of the emitted light beam reflected by the reflective surface;and determining a plurality of reflection angles for the reflected lightbeam, wherein each of the plurality of reflection angles is associatedwith one of the plurality of angles.
 8. The method of claim 1, furthercomprising: providing a drive mechanism configured to rotate the portionof the cartridge about an axis; and; coupling the drive mechanism to thecartridge; wherein the step of producing the rotation over a pluralityof turns, the rotation is produced by the drive mechanism.
 9. The methodof claim 8, wherein: in the step of providing the drive mechanism, thedrive mechanism includes a motor and a first belt member; and furthercomprising a step of operably coupling the motor and the cartridge bythe first belt member.
 10. The method of claim 9, wherein in the step ofproviding the drive mechanism, the drive mechanism further includes asecond belt member coupled to the first belt member by a first pulleyassembly.
 11. A coordinate measurement device comprising: a cartridge; afirst bearing coupled to the cartridge; a second bearing arranged in apredetermined relationship with the first bearing to define an axis; anangle measurement device that is operable to measure an angle ofrotation of a portion of the cartridge about the axis and transmits anangle measurement signal in response; a rotational counter that isoperable to measure a second rotational value of the cartridge, thesecond rotational value being a third number of turns the portion of thecartridge is rotated in the forward direction minus a fourth number ofturns the portion of the cartridge is rotated in the reverse direction;a processor that is responsive to executable instructions when executedon the processor for measuring for each of a plurality of turns of thecartridge a plurality of first rotational values, each of the pluralityof first rotational values comprising a first number of turns of theplurality of turns that the portion of the cartridge is rotated in theforward direction minus a second number of turns of the plurality ofturns the portion of the cartridge is rotated in the reverse direction;wherein the processor is further responsive for determining a firstplurality of displacements at a first position along the axis, each ofthe first plurality of displacements being associated with one of theplurality of angles and one of the plurality of first rotational values;and wherein the processor is further responsive to determinecompensation values based at least in part on the measured plurality ofangles, the plurality of first rotational values, and the determinedfirst plurality of displacements.
 12. The device of claim 11, whereinthe processor is further responsive to measuring a three-dimensionalcoordinate of an object based at least in part on the angularmeasurement signal, the stored compensation values, and the secondrotational value.
 13. The device of claim 12, wherein the processor isfurther responsive to measure a second plurality of displacements at asecond position along the axis, each of the second plurality ofdisplacements being associated with one of the plurality of angles, thesecond displacements being measured along a second line substantiallyperpendicular to the axis, wherein the first position and the secondposition are separated by a first distance.
 14. The device of claim 13,wherein the determining compensation values includes the processordetermining the compensation values further based at least in part onthe second plurality of displacements and the first distance.
 15. Thedevice of claim 11, further comprising: a test apparatus removablyattached to the cartridge, the test apparatus having a first sensor anda second sensor; and wherein in the measuring of the plurality of firstdisplacements, the first displacements are measured with the firstsensor, and in the measuring the plurality of second displacements, thesecond displacements are measured by the second sensor.
 16. The deviceof claim 11, further comprising: an autocollimator; a reflective surfaceattached to one end of the cartridge and configured to rotate about theaxis with the portion; wherein the processor is further responsive todetermining a plurality of reflection angles in response of light beamemitted by the autocollimator and reflected off of the reflectivesurface, wherein each of the plurality of reflection angles isassociated with one of the plurality of angles.
 17. The device of claim11, further comprising: a drive mechanism coupled to the cartridge andconfigured to rotate the portion of the cartridge about an axis; whereinthe producing the rotation over a plurality of turns, the rotation isproduced by the drive mechanism.
 18. The device of claim 17, wherein thedrive mechanism further comprises: a first belt member; and a motorcoupled to the cartridge by the first belt member.
 19. The device ofclaim 18, the drive mechanism further comprises a second belt membercoupled to the first belt member by a first pulley assembly.